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A better workshop foundation?

Had a guy all set up to make a compacted gravel base for my new 12x20 Amish shed. Then I had 2 more guys show up to bid on the job. Both told me they thought compacted gravel was a bad idea, even with an 8x8 treated frame made in 3 sections with common sides. Some settling was inevitable they both believed. The first guy was just starting to tell me of a better way using sonotubes and pouring concrete "pads" down 30" and leveled perfectly with on another when a second guy showed up and agreed that he thought that would be far more stable. The first guy said he thought just one in each corner for the skids to sit on would suffice.
I did not care for his suggestion and he soon left as I was discussing with the 2nd guy what he proposed. It was his opinion that 4 along each outer skid and 2 more under separate skids near the center of the building.
I'm convinced now his plan is the way to go and I'd like to get some opinions from members here on that method.
He told me the cost would probably be fairly close to a framed compacted gravel pad, or perhaps a bit less.
Please share your thoughts on this idea.
Also, all the while thinking sound deadening it occurred to me that a hard rubber pad on top of these structures might help isolate sound transmission. Any thoughts on this idea?
The shop will have 4x6 skids notched 1" for each 2x4 joist placed 12" on center and a 3/4" plywood floor.

I did concrete work for 30 years. Gravel will settle. How much depends as much on how it is put in as it does on the type of graveland the base you put it on. Mostly there are a lot of voids in gravel, and the weight of it and what ever goes on top of it will push it down into the dirt and/or sub grade. They do use a fabric type of mesh under some roads now which kind of acts like a big snow shoe. That helps some. You can put a lot of fines (sand and/or dirt) on top and wash that down with a lot of water and it makes it more solid, but even gravel roads get ruts in them from constant use. By the time you do the sono tube route, you may as well go ahead and pour a 4 inch slab. The slab, again, works like a big snow shoe. You may do okay with a smaller slab, but having concrete to sit the shop on is the most solid way to go. Plywood will flex. As far as sound deadening, not much noise from turning wood...
robo hippy

I did concrete work for 30 years. Gravel will settle. How much depends as much on how it is put in as it does on the type of graveland the base you put it on. Mostly there are a lot of voids in gravel, and the weight of it and what ever goes on top of it will push it down into the dirt and/or sub grade. They do use a fabric type of mesh under some roads now which kind of acts like a big snow shoe. That helps some. You can put a lot of fines (sand and/or dirt) on top and wash that down with a lot of water and it makes it more solid, but even gravel roads get ruts in them from constant use. By the time you do the sono tube route, you may as well go ahead and pour a 4 inch slab. The slab, again, works like a big snow shoe. You may do okay with a smaller slab, but having concrete to sit the shop on is the most solid way to go. Plywood will flex. As far as sound deadening, not much noise from turning wood...
robo hippy

A 5 hp DC in a 12 by 20 shed??? That size of DC would do fine in a 2000+ sq. ft. shop, maybe even bigger if it is a good system. Lathe on concrete slab. The DC, depending on what type and style (2 stage?) would do okay on some of the 12 X 12 inch pavers, but I would want it anchored to the walls some how. Most of the noise from a DC is from the impellor blades and air flowing through the pipes, not from machine vibration.
robo hippy

With the shed on a slab wouldn't the noise from just the impellor increase? Wouldn't every noise inside the shed be louder outside with the shop on a slab?

The plywood floor will transmit much more sound than concrete. Imagine a piano with a concrete soundboard or an acoustic guitar with a cast concrete body. Coupling sound vibrations to air takes a lot of motion and plywood will flex more easily than concrete.

Beranek's Law:

It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.

Pouring a slab may have local zoning implications...check on that before deciding between a slab or the concrete supports. A slab would be "more permanent" than posts. From a cost perspective, the concrete posts "may" be less expensive and you can still put in geo-textile covered with compacted stone to keep things clean under the shed if you want.

So how many sonotube piers do you think would suffice?
One bidder said 1 in each corner would work what with the 12" on center 2x4 joists. Another recommended 4 on each outer skid and one under each of the 2 inner skids near the center of the 20' span.
I called the Amish builder and he strongly suggested 4 under each outer skid (there are 4 skids) and 3 under the inner most 2 skids.
Just called the guy wanting to use just one sonotube,in each corner. I had called him because after more than a week I was wondering why I had not received his bid. He told me he was waiting on me to make up my mind.
The contractor suggesting 4 on the outer skids,plus 2 more in the middle of the 2 center skids has not gotten back to me either.

Would members here please give me their honest opinions as to making a foundation using sonotubes.
They have not started building my shed yet though I ordered it nearly 5 weeks ago.
I am having a lot of trouble understanding the vast differences between bidders and I'm about to the point of doing the sonotubes myself.
Need advise, please. Bill

The shed will be used to store 2 600lb motorcycles, a Grizzly g0766 lathe, more than likely more woodworking machines in the future such as a good band saw. Also will be using for the rest of my tools, miter saw, heavy bench and vice, small table saw and storage for many other things. May also move my reload bench to the shed which weighs about 250lb with press.
If I go with slab I'll have to deal with additional codes. But I've not ruled that out. I will also be storing sveral hundred pounds of kiln dried wood.

If you go the sono tube route, for a 12 by 20 foot shop, then minimum one in each corner, one in the middle of the 12 foot side, and 2 in between the corners on the 20 foot side. They would need to be pretty much dead level with each other. Easier to pour a strip footing, maybe 12 inches wide, and 6 inches deep. You could get away with a full 4 inches deep. Rebar, one piece minimum around the perimeter. 2 pieces is better. It does spread the load, and keeps it together when it cracks. Only 2 guarantees you can give about concrete is that one it will crack, and two, no one is going to steal it... Look into the cost of a full slab, 2 by 6 outside frame box, slab about 4 inches thick. Rebar 2 ft. on center, not mesh (it always seems to be on top of the slab when you are finishing it, or down in the gravel. With sono tubes, you can get a bunch of bags of fence post mix, and it will work, but it is a lot of work to do. If you get one of the portable mixes on site dispatch trucks, if they can get back to it, best value is a strip footing or a slab. Some even have chutes that will swing back and forth to place the concrete on the ground. Don&#39;t try to pour it yourself unless you know concrete or it will work you half way to death, and you are stuck with the final product. From what you are going to put into the shed, it sounds like it is about half the size you will need unless every thing is moveable, and every time you want to go work in the shop you have to move a bunch of stuff out of the way. Out where I am, you can do 200 sq. ft. without having to get a permit. No idea what code is where you are. Some times a car port can be done with slab and no permit. They may require one side to be open and the other 3 can be enclosed. I am limited on my lot to any out building having a maximum size of 10% of the total surface area of the lot. I had to cheat and put in a small &#39;living area&#39; in the shop. I may rent or use it for office space. Hope this helps some....
robo hippy

Personally I would go the sonotube route as your contractor suggested. Its the strongest and most cost effective way to go. Having a wood floor is the most comfortable in the long run, but mass does a lot to dampen noise so if sound is a higher priority than cost and/or comfort get a concrete slab. If you do an all wood floor, add another layer of 3/4" plywood with a layer of hardwood floor underlayment (dense roll foam) between the two plywood layers. This added layer adds mass and rigidity. The foam isolates vibrations and also acts as a vapor barrier from the ground. Your back, knees and tools will appreciate this option in the long run!

Well since it's been 5 weeks since I put down the downpayment i'm sure I could make it longer, but I cannot go beyond 12' wide and still get it to the back yard.
This whole thing has been very stressful and i'm almost to the point of putting in sonotubes myself.

A good source for information may be your local permit office. They can let you know how deep to place the sonotubes to avoid heaving from frozen soil, if that is likely to happen in your area. They may also help with load bearing calculations to address the number needed and locations.

If available in your area look into helical piles, sometimes called screw piles. Machine comes and winds them into the ground until the correct resistance is achieved. Then they cut the tops off and attach a fitting with a plate or saddle on top for the beams to sit on. They are in and out in an hour or two. Not the cheapest but they will be ready when your shop gets to the site.